1. Field of the Invention
The present invention relates to medical devices for monitoring patient respiration.
2. Description of the Prior Art
There has long been a need for a medical monitoring device to reliably determine when breathing of a patient ceases. The medical term for this phenomenon is apnea. Since a patient at risk cannot be personally observed at all times, a device is needed to alert medical personnel at times when breathing stops or slows excessively.
It is known that impedance across electrodes mounted on the skin varies with chest motion. The motion of breathing can therefore be detected by sensing such impedance changes. Previous monitors detect each such change as a breath. When the changes do not occur, the devices alert the medical personnel of cessation of breathing.
Prior art monitors have a time period set for the acceptable period during which no breaths are sensed. If the time period is exceeded, an alarm is sounded.
One problem in this sensing is that the range of change in impedance due to breathing overlaps the range of change of impedance due to the mechanical contraction and surge of blood flow resulting from heart beats. Therefore, a heart beat may be mistakenly interpreted as a breath. This unwanted signal is known as a cardiac artifact.
It is desirable to reject these artifacts so they are not erroneously interpreted as breaths. If an artifact is counted as a breath, the apnea timer is restarted. Therefore an apnea alarm may be delayed or not sound at all.
Prior art cardiac artifact rejection, sometimes called coincidence, is based upon cardiac artifact occurring at the same rate as ECG during an apnea. The cardiac artifact is actually not coincident with the electrical ECG signal because it is caused by the physical contraction of the heart and the surge of blood flowing through the body with each contraction which is delayed from the electrical ECG event. There is, however, a one for one correspondence between an ECG and the cardiac artifact which follows.
These prior monitors assume that breathing rarely occurs at the same rate as the heart rate or, it if does, is not sustained for a long period of time. If a prior art monitor begins triggering on cardiac artifact during an apnea, the apparent breath rate will be equal to the heart rate. Based upon this assumption, the prior art devices override the breath detects and invalidate the breath detects until the rates of apparent breaths and heart became unequal. This results in an apnea alarm at the end of the set period.
Prior art monitors depend on the fact that the amplitude of cardiac artifact is either smaller than the monitor's detector sensitivity or large enough that the monitor's detector can consistently detect the artifact. If the amplitude of the cardiac artifact signal is close to the threshold of the breath detector, the breath detector may not consistently detect the cardiac artifact and the apparent breath rate will not be equal to the heart rate. In this case the device cannot make the differentiation between the cardiac artifact and actual breaths. The device may treat some of the ECG artifact as breaths. This results in either a delay or a lack of alert, and a possibly dangerous delay in medical attention.
Examples of the operation of prior art devices appear in FIGS. 1a-c At the top of FIG. 1a is shown the ECG printout, including QRS complexes Q.sub.0 -Q.sub.8, and ECG detection triggers ED.sub.0 -ED.sub.8, which are issued for each QRS wave complex in the ECG. Using detection triggers ED.sub.0 -ED.sub.8 prior art monitors have measured average ECG rate and intervals ECG.sub.0 -ECG.sub.8 between triggers.
In Case A of FIG. 1a a normal breathing pattern is shown. Also shown are breath detect triggers BD.sub.a1 -BD.sub.a3 for the three detected breaths. Between each breath detect trigger BD.sub.a1 -BD.sub.a3 is shown a respiratory interval R.sub.a1 -R.sub.a3. In this case, respiratory intervals R.sub.a1 -R.sub.a3 are not equal to the average heart intervals, shown by intervals ECG.sub.1 -ECG.sub.8. Additionally, the interval R.sub.a1 is not equal to the previous ECG interval ECG.sub.1. Consequently, breath detect BD.sub.a1 is accepted as a valid breath by the monitor.
Case B of FIG. 1b illustrates a large cardiac artifact during an apnea. Breath detects BD.sub.b1 -BD.sub.b8 are triggered on each artifact. In this case the average breath interval, shown by intervals R.sub.b1 -R.sub.b8, is equal to the average ECG interval, shown by intervals ECG.sub.1 -ECG.sub.8. Also interval R.sub.b1 is equivalent to interval ECG.sub.1, as are intervals R.sub.b2 -R.sub.b8 equivalent to intervals ECG.sub.2 -ECG.sub.8. A prior art monitor with cardiac artifact rejection based on average breath and heart interval equivalency will invalidate breath detects in Case B of FIG. 1b after a certain number of these cardiac artifact detects. Some prior art monitors test for equivalency between the previous respiratory interval and the previous ECG interval. These monitors may disqualify the breath detect following a single cardiac artifact.
Moderate cardiac artifact during an apnea is shown in Case C of FIG. 1c. In this case the amplitude of the cardiac artifact is very close to the threshold of the breath detector of the prior art monitor. For this reason, not every cardiac artifact is detected. The average breath interval, shown by intervals R.sub.c1 -R.sub.c4, is not equivalent to the average heart interval, shown by intervals ECG.sub.1 -ECG.sub.8. Also, interval R.sub.c1 is not equivalent to interval ECG.sub.1. Interval R.sub.c2 is not equivalent to interval ECG.sub.3, but interval R.sub.c3 is equivalent to interval ECG.sub.4. Prior art monitors based on equivalent average intervals will not invalidate any of the three cardiac artifact detects. Prior art monitors based on equivalent previous intervals will not invalidate cardiac artifact detects at breath detects BD.sub.c1 and BD.sub.c2, but may invalidate the artifact at detect BD.sub.c3.
Examples of prior art monitor performance are shown in FIG. 2. FIG. 2 illustrates simulated signals actually used to test prior art monitors and the present invention. FIG. 2 illustrates quiet breathing at 30 breaths per minute, from time t0 to time t1, followed by a large 4 ohm breath sigh. The one-minute apnea ends at time t2, when quiet breathing resumes. The illustration includes a cardiac artifact at 0.25 ohms at the same rate as the ECG, 90 beats per minute.
Graphs of sensing of prior art monitors A-C, and the present invention, have a vertical bar for each event which is detected and validated as a breath by the monitor.
Prior art monitor A uses the breath and ECG rate averaging method of rejecting cardiac artifact. When the average breath rate reaches within a certain percent of the ECG rate, cardiac rejection circuitry invalidates the breath detections until the respiration rate is less than a certain percent of the ECG rate.
Prior art monitor A detects once for each breath at 30 breaths per minute from time t0 to time t1, and once for the sigh at time t1. Approximately six seconds following the sigh, monitor A begins detecting cardiac artifacts without invalidating them. After five cardiac artifact detects the supposed respiration rate approached the heart rate and the circuitry began invalidating the breath detects for approximately six seconds. During this period monitor A stopped consistently detecting every cardiac artifact. This is because the amplitude of the artifact is just at the sensitivity of the breath detector. Very slight changes in artifact amplitude as the base line is approached, result in inconsistent breath detections so that the supposed breath rate falls below the average heart rate. This causes the cardiac artifact rejection circuit to stop invalidating the spurious breath detects, allowing some false detects to be treated as breaths during the time during which the signal returns to baseline. As the signal reaches the baseline, the respiratory detector sensitivity becomes just sensitive enough to consistently detect the next seven cardiac artifacts. At this time the cardiac artifact rejection circuit finally locks out the breath detections and the monitor begins measuring the apnea period, 40 seconds after it began.
Prior art monitor B compares a single breath detection interval with a single ECG detection interval. As with monitor A, breath detection sensitivity is just at the level which detects the 0.25 ohm cardiac artifact, resulting in inconsistent intervals between breaths. When these intervals are compared with the corresponding ECG intervals, there are large differences at times. When this occurs, the cardiac artifact is occasionally accepted as a valid breath. The result is that monitor B never reaches the 15 second apnea alarm time and no alarm occurs.
Prior art monitor C has no cardiac artifact circuitry. Every cardiac artifact greater than the sensitivity of the breath detector is assumed to be a breath. In this example, all artifacts were of sufficient amplitude to be incorrectly sensed as breaths. No apnea alarm sounded, since no cessation in breathing was detected.
Another manifestation of alarm failure results from switch errors and failures. Some attempts have been made to make alarm limit switch settings tamper resistant. One approach is to hide switch settings behind a trap door that is not easily opened. This approach makes it difficult to quickly review switches settings at a glance. This method also does not guarantee that the trap door has not been opened and switches changed.
Another approach is to sound an alarm if accessible switches are changed, while the unit is operating, without simultaneously pressing the reset button. This somewhat safeguards tampering by an anauthorized operator. This method and the rest of the prior art does not address alteration of switch settings while the unit is off. Apparatus is needed for detection and warning of unauthorized switch setting alteration.
Additionally, switches are prone to failure. Failure can be due to the user positioning a switch between two settings such that the contacts for both settings either short together or such that neither contact is closed. This may result in both settings being sensed or in neither setting being sensed. Failure can also be mechanical such that one or more switch position contacts are shorted together or are open and do not make contact. The result of such mechanical failure may provide misleading input to the circuitry. These particular switch settings may become undefined in meaning, such as rate or length of apnea period is unclear or unknown. Thus the patient being monitored may not be monitored with appropriate alarm limits, resulting in a dangerous condition. What is needed is a way to verify that switch contacts are neither shorted nor open and are mechanically reliable.
What is needed is a reliable monitor which can differentiate between actual breath signals and cardiac artifacts. Additionally a device is needed which has both tamper prevention features and fail-safe feature in case of problems, so that alerts are not delayed or missed.